In the typical radio backhaul networks as shown in FIGS. 1 and 2, two all outdoor radio units, a working unit (11 in FIG. 1 or 21 in FIG. 2) and a protection unit (12 in FIG. 1 or 22 in FIG. 2), are deployed in a (1+1) redundant configuration to protect against hardware failure, including transmitter module failure. Note that the working unit (11 or 21) and the protection unit (12 or 22) can be connected to the same antenna as shown in FIG. 1 or can use two separate antennas as shown in FIG. 2. The latter case has the added benefit of protecting against selective path fading in the receiving direction. For illustrative purposes, the two figures depict only components relevant to the transmitter module protection. In both cases, the customer data traffic is received by the digital data interface (113 or 213) in the working radio unit (11 or 21) and then transmitted to modems in both the working radio unit (11 or 21) and the protection radio unit (12 or 22). In other words, the working unit (11 or 21) is the unit that receives the customer data traffic directly from the user's equipment, and whose transmitter is online when there is no failure. The protection unit (12 or 22) does not receive the customer data traffic directly from the user's equipment. Instead, the protection unit (12 or 22) receives the customer data traffic from the working unit (11 or 21) via the protection interface (112 or 212) in the working unit (11 or 21), a cable interconnect between the two units, and the protection interface (122 or 222) in the protection unit (12 or 22). In case of a failure detected at the transmitter (115 or 215) in the working unit (11 or 21), which will be muted accordingly, the transmitter (125 or 225) in the protection unit (12 or 22) will be un-muted. When this happens, the protection unit (12 or 22) still receives the customer data traffic from the working unit (11 or 21) via the cable interconnect as described above.
Using the (1+1) redundant configuration in FIG. 1 as an example, although the output signal that carries the same data traffic is connected to both transmitters 115 and 125, only one of the two transmitters, either the transmitter 115 in the working radio unit 11 or the transmitter 125 in the protection unit 12, is online when there is no failure. The transmitter 125 in the protection radio unit 12 is muted (as indicated by the “X” symbol 125-9). When the radio control logic detects failure of the online transmitter 115 in the working radio unit 11, it mutes the failed transmitter 115 and un-mutes the transmitter 125 in the protection radio unit 12, so as to restore the customer data traffic flow. As such, each of the two participating units can be designated as a working unit or a protection unit. The dashed lines in FIGS. 1 and 2 represent the connections that are not carrying the customer data traffic at the moment, but may carry the customer data traffic if the definitions of the working and protection units are reversed.
Note that there are small but random frequency and phase offsets between a pair of working and protection radio units, which may cause incoherency between the two respective carrier signals. Therefore, it is necessary to mute one of the radio units in the pair. If both transmitters were online with the random frequency and phase offsets, the two carrier signals each would be a high level noise to the other and the receivers on the remote side of the radio link will not be able to recover the transmitted data correctly.
On the other hand, the switch from the working radio unit to the protection radio unit results in a traffic interruption. Currently, the industry-standard traffic interruption is less than 50 ms. But it is difficult to meet this requirement for all possible transmitter failures. It is even more difficult to meet this requirement in the all outdoor radio because of the additional time needed to communicate between the controllers in the pair of radio units. Moreover, because the protection radio unit is muted, half of the transmitting power according to the current transmitter protection scheme is not used at any given time.